I’m pretty sure the arguments boil down to you each having two different points of view, then both sides finding evidence to support it (which is totally normal, by the way). As you said:
I am not going to try to address the implied moral logic of van Onselen: that the criterion upon which we ought to judge immigration policy is the welfare of incumbent residents. As it happens, I think this is a deeply unethical social welfare function.
This is the crux. I know enough economics now to know that you can get just about any answer from technical assessments of marginal welfare effects. So let’s leave that to the side. Let’s stop pretending technical assessments will provide an answer or change minds.
I personally think this ideological different is strange, and am curious as to your ethical viewpoint. For example, if you think nations are a useful organisational institution, and democracy is a useful way to offer some checks on power structures in a nation, surely you imply support for judging national policy based on the welfare of residents, as that is the underlying rationale of the democratic nation-state.
If you don’t, then you open up some puzzles. For example, if you are instead a global welfare maximiser, start sending ships to collect the neediest people from Bangladesh, or the drought-affected regions of Somalia, South Sudan, Tanzania, Mozambique etc., and bring them over. It’s the easiest way to do it. Or just send them free food and equipment. Our policy settings, and most individual choices, are completely unlike the choices of a global welfare maximiser.
In any case, any policy position apart from complete open borders is a de facto population policy. The question is whether the benefits outweigh the costs (to whoever, based on your ethical view) at an immigration rate of 200,000+ per year, or whether the net benefits are higher with a rate <100,000 per year. Judging by the comments and my experience talking to people from all across the country, the common view is that the benefits are more apparent with lower rates.
If we take some counterfactuals for a moment, consider if this debate was happening in the early 2000s when the immigration rate was about half what it is now. I assume you would argue that higher is better. I have no idea what Leith would argue, since I don’t think he thought it was an issue back then.
Then immigration doubles. You, I assume, would still be happier with a higher rate. Leith now sees it as a problem.
Let’s double the rate again (to half a million a year). Would your position change? I suspect not. I suspect, again, that this argument is really about conflicting underlying moral and philosophical viewpoints.
Anyway, my feeling is many of the policy failures you discuss are also much easier to remedy with lower rates of immigration.
Lastly, if you genuinely want higher immigration you should be ignoring this debate and not fuelling the fire, since there is massive popular support for lowering immigration back to pre-2006 levels, and the more it is in the media, the more politicians will have to respond to this groundswell.